Additional studies in extracts of tissues of rodents, Sprague-Dawley rats, ob/ob mice and their thin littermates, control and diabetic Chinese hamsters, have confirmed our previous findings of the presence of an insulin-like material. Although brain and liver membrane receptors are influenced by circulating plasma insulin, the insulin-like tissue content of the same tissues are not influenced. The status demonstrating the presence of insulin receptors in rodent brain and liver membranes were extended to chicken embryos. Insulin receptors are present in membrane preparations of heads and bodies of chick embryo by 4 days of incubation. Binding of labeled insulin to brain and liver membranes from 8 to 18 day embryos increases with time of development. Recently, insulin-like growth factor binding was studied in chick embryo tissue. Using labeled multiplication stimulation activity peptide there appears to be specific insulin-like growth factor receptors on brain membrane preparations of embryos as early as day 2 of development. The growth factor finding pattern is different than that for insulin binding in chick embryo. Insulin receptor structure studies have been extended in rat brain and rat brain microvessels. The gel electrophoresis data indicate that the insulin receptor in whole brain homogenates is distinguished from those in peripheral tissues by structural alterations including changes in the carbohydrate moiety of the receptor. The microvessel insulin receptor exhibits features typical of liver membrane and atypical of whole brain homogenates. Microvessle and liver membrane preparations from rats made diabetic with streptozotocin showed changes in receptor number (up regulation) which is in contrast with whole brain receptors which showed no change.